In the field of wireless communication systems, cellular systems using licensed (frequency) bands, which are typically dedicated for wide-area communications, and non-cellular systems using unlicensed (frequency) bands, which are typically dedicated for local-area communications, have been developed and are still under developments in parallel. In view thereof, an issue in this field resides in enabling a beneficial and efficient usage or cooperation of both coexisting types of wireless communication systems.
For example, aforementioned cellular systems could involve any wide-area cellular communication standard such as GSM, GPRS, UMTS, IMT-A, LTE, LTE-A, or the like, and aforementioned non-cellular (or, as referred to hereinafter, unlicensed) systems could involve any local-area communication standard such as IEEE802.11 (also referred to as Wireless Local Are Network), IEEE802.16 (also referred to as Worldwide Interoperability for Microwave Access), WiFi, or the like.
In the following, for the sake of intelligibility, LTE including LTE-Advanced (Long-Term Evolution according to 3GPP terminology) is taken as a non-limiting example for cellular systems and WiFi is taken as a non-limiting example for unlicensed systems, which are applicable in the context of the present invention and its embodiments. However, it is to be noted that any kind of wireless communication systems may likewise be applicable for any one of the cellular and unlicensed systems, as long as exhibiting comparable features and characteristics as described hereinafter.
Nowadays, unlicensed bands are widely used for wireless local-area communications, especially in the worldwide deployed WiFi systems. Because no spectrum licensing costs are involved, overall cost of a local network operating in an unlicensed band is much lower than the general cellular communication system. Moreover, although the unpredictable interference may degrade the system performance in an unlicensed band, the wide range of (at least some of) the available unlicensed bands provide the potential to avoid nearby interference and to maintain the traffic flow. As shown in Table 1 below, for example, there are 100 MHz of unlicensed bandwidth in the 2.4 GHz band and 150 MHz of unlicensed bandwidth in the 5.8 GHz band, both of which is large enough compared with 20 MHz of licensed bandwidth of general LTE systems.
Accordingly, any specified ISM band with a sufficiently large unlicensed bandwidth is generally applicable for wireless local-area communication systems which are applicable in the context of the present invention and its embodiments.
TABLE 1Unlicensed ISM bandsFrequency rangeCenter frequency6.765-6.795MHz6.780MHz13.553-13.567MHz13.560MHz26.957-27.283MHz27.120MHz40.66-40.70MHz40.68MHz433.05-434.79MHz433.92MHz902-928MHz915MHz2.400-2.500GHz2.450GHz5.275-5.875GHz5.800GHz24-24.25GHz24.125GHz61-61.5GHz61.125GHz122-123GHz122.5GHz244-246GHz245GHz
Given the wide range and bandwidth of available unlicensed bands, an issue resides in how to utilize them efficiently. In the context of the present invention and its embodiments, it is specifically considered hoe the unlicensed bands could be beneficially and efficiently utilized for cellular offload, especially when terminals are equipped with both a cellular (e.g. LTE) radio and an unlicensed (e.g. WiFi) radio.
A conceivable approach in this regard could be to offload traffic from a cellular (e.g. LTE) system, i.e. the licensed or cellular band, to a local or unlicensed (e.g. WiFi) system, i.e. the unlicensed or non-cellular band. In such approach, which could be referred to as WiFi offloading, a terminal needs to support WiFi MAC and PHY functions besides LTE MAC and PHY functions in order to be capable of properly communicate in both systems, i.e. bands. This would however increase the hardware and software complexity of the terminals. Also, due to the independent processing of radio signals between LTE and WiFi, the unlicensed band, i.e. the spectrum thereof, cannot be utilized in an efficient manner.
Being different from licensed bands, unlicensed bands are shared by all kinds of license-free applications. Hence, an unlicensed band generally has stricter requirements on transmission power and outband emission than a licensed band. Also, unlicensed-band communications, i.e. the terminals performing communications in unlicensed bands, must be able to tolerate any interference from other possible equipments.
Carrier sensing is typically applied in WiFi systems as the major technique to avoid unpredictable interference in the unlicensed band. In WiFi systems, the so-called CSMA/CA technique is used, and any station (including terminals and base stations, etc.) will only transmit data after the carrier channel is sensed and no signal is found in that channel. In this regard, carrier sensing in CSMA is not only used for external interference avoidance, but is also used as a multiple access mechanism. That is, when a station has found by carrier sensing that another station is transmitting traffic on a particular channel, it will postpone its transmission until the channel becomes available again. Although such a distributed sensing and access mechanism automatically makes the system robust to external interference, it requires a long random backoff delay to avoid contention between multiple access users, i.e. stations. Moreover, as mentioned above, using WiFi directly in the cellular offloading increases the hardware and software complexity at terminal side as any terminal would have to support WiFi MAC and PHY functions besides LTE MAC and PHY functions.
Hence, the aforementioned approach for offloading cellular traffic to an unlicensed band is not preferable in terms of its requirements on the terminal side and its overall performance.
In view of the above, an important issue for traffic offloading using an unlicensed band resides in how to realize interference tolerance to external environments for (cellular) terminals specifically designed and dedicated for performing communications in a licensed or cellular system (band). Another an important issue for traffic offloading using an unlicensed band resides in how interference can be reduced, or at least excessive interference can be avoided, in view of both the unlicensed nature of (and, thus, quasi unrestricted access to) the band used for traffic offloading and the mobility of terminals in the context of wireless communications.
Conventionally, there are no solutions to overcome such problems for traffic offloading using an unlicensed band in an efficient manner.
In view thereof, there do not exist any mechanisms or techniques of spectrum sharing for traffic offloading using an unlicensed band. More specifically, there do not exist any mechanisms or techniques of spectrum sharing to enable traffic offloading from a cellular frequency band to an unlicensed frequency band with reduced interference.
Thus, there is still a need to further improve such systems in terms of proper spectrum sharing measures in this regard.